The present invention relates to gyroscopes. Specifically, the invention relates to gyroscopes and their signal processing electronics.
The interferometric fiber optic gyroscope (IFOG) is an established technology for accurately measuring angular rotation. Because the IFOG is an optical, solid state design with no moving parts, it can be used for long life, high reliability applications such as land vehicle navigation.
Requirements for a gyroscope intended for use in land navigation systems with coupled dead-reckoning (DR) and GPS (Global Positioning System) input are governed more by cost than by performance considerations. The gyro is used as a gap filler for those systems where no outage is permissible. The GPS data can then be used to periodically correct the dead reckoning sensors, reducing the demands on each. The cost of this type of land navigation system is heavily dependent on the cost of the gyroscope employed. Although the wide performance range of the IFOG makes it well suited for applications such as land navigation, further cost reduction in the gyroscope optical configuration and electronic signal processing is required to make this technology economical to use for many systems such as land navigation systems.
The fundamental working principal behind the IFOG is the Sagnac effect. In this effect, two counter-propagating light waves traversing a loop interferometer acquire a phase difference when the loop is rotated about its axis. The IFOG uses fiber optic components to form the Sagnac interferometer. Accurate measurement of the Sagnac phase difference induced by rotation requires the parasitic phase differences, which can vary with environment, be suppressed. For this reason the principal of optical reciprocity is used to select portions of the counter-propagating waves which pass through the interferometer along a common path. Variations of the system by the environment changes the phase of both waves equally and no difference in phase delay results; the sensor is environmentally stable. The use of optical reciprocity in the IFOG architecture results in what is commonly referred to as the xe2x80x9cminimum configuration.xe2x80x9d
In the minimum configuration (MC) IFOG the light is emitted from a semiconductor or rare-earth doped fiber light source, passes through the first coupler where half of the light is dissipated, and half is sent into the interferometer through the polarizer. A second coupler splits the light into two approximately equal intensity, counter-propagation beams which traverse the coil. The two light beams then recombine at the second coupler where they interfere. This combined light beam then passes through the polarizer a second time in the opposite direction, and half of the light is directed to the detector by the first coupler. The first coupler is not part of the optically reciprocal Sagnac interferometer. Its sole purpose is to direct some of the returning light into a photodetector and to minimize direct coupling of light energy from the source to the detector. To maximize the optical power incident on the detector, the optimum splitting ratio of this coupler is 3 dB. This leads to an inherent 6 dB of system loss since this coupler is passed twice; it is independent of the coupler insertion loss.
In order to reduce the optical configuration complexity and cost, yet maintain the principal of reciprocity, a xe2x80x9creduced minimum configurationxe2x80x9d is used. In the reduced minimum configuration (RMC) IFOG, the first coupler has been removed and the interferometer output is read out through a detector positioned at the back facet output of the light source. The light passes through the source before being received by the detector. The RMC gyroscope maintains the principal of optical reciprocity since the light in the interferometer still traverses a common optical path. The inherent system loss of 6 dB from the first coupler is eliminated. Also, depending on the type of light source chosen, and the drive current operating range, the source can act as an optical amplifier for the returning light. Therefore, the signal-to-noise ratio of the RMC gyroscope is as good, and potentially can be better than the conventional MC gyroscope design. Many low cost laser diode packages contain a back facet photo-detector. Thus, the detector is provided by the laser diode manufacturer and the cost of purchasing a separate detector is eliminated in this design. Also, the equipment and labor needed to align the first coupler output fiber to a separate detector is eliminated. The detector is aligned to the back facet by the manufacturer of the laser diode. When the input fiber pigtail is aligned to the optical source, the output is automatically aligned to the detector in the same operation. The RMC also eliminates two fiber-to-fiber fusion splices, further reducing the optical assembly cost.
A piezo-electric transducer (PZT) is used in both the MC and RMC IFOGs to modulate the phase difference between the two counter-propagating light beams. Other methods of modulating the phase difference, for example, electro-optic material such as lithium niobate can be used. This phase modulation serves two purposes. One is to move or bias the interferometer to a more sensitive operating point and also allow the determination of rotation sense. The other is to move the detected signal from direct current (DC) to alternating current (AC) in order to improve the accuracy of the electrical signal processing. With sinusoid phase modulation, the interferometer output signal is a infinite series of sine and cosine waveforms whose amplitudes are Bessel function related to the phase modulation amplitude. The fundamental signal is at the applied modulation frequency with subsequent odd and even harmonic signals. Many signal processing approaches have been proposed which use the ratio of the fundamental and the next three lowest order harmonic signals amplitudes to detect rotation rate while at the same time maintaining a stable, linear output scale factor. However, implementation of these approaches in analog and/or digital electronic hardware is complex and expensive. Also, the use of the light source as both a light emitter and amplifier is not without problems. Distortion of the interferometer signal can occur due to traversing the light source prior to detection and due to bandwidth limitations of the back facet photo-detector. The signal harmonic amplitudes can be altered by these effects leading to a measured output rotation scale factor error if the RMC gyroscope design is used with the conventional harmonic ratio signal processing methods. This is a significant drawback in multiharmonic processing methods. Therefore, a much simpler signal processing design, which is not affected by an error in the relative amplitude of the gyroscope harmonic signals is desired.
Scale factor linearity (i.e. measured output rate versus input rate applied) is maintained due to the intrinsic linearity of the Sagnac effect for low rotation rates and is a sinusoid for larger rates. However, the more difficult problem is maintaining a constant scale factor during environment changes (i.e. temperature, vibration, etc.) and over the life time of the sensor.
It is an object of the present invention to provide an IFOG signal processing system which performs well for both the MC and RMC gyroscope designs.
It is a further object of the present invention to provide an IFOG signal processing system which is simple and low-cost to produce.
It is a yet another object of the invention to provide an IFOG system which accurately determine rotation rate of the sensor coil.
It is still another object of the invention to provide an IFOG system which maintains a constant scale factor during environmental changes.
It is a further object of the invention to provide an IFOG system with simplified signal processing electronics and which eliminates non-essential optical components and splices.
The foregoing objects are provided by an improved IFOG system which comprises a fiber sensing coil; a semiconductor or rare-earth doped fiber light source emitting light with an associated light source intensity, said source having a front output and back output; an optical coupler attached to said front output for receiving said light from said light source, said coupler creating two substantially equal intensity light beams for simultaneous transmission into said sensing coil said coupler attached to said coil; wherein said fiber sensing coil supplies return light to said coupler from said equal intensity light beams and said coupler combines and interferes said return light into a combined light beam; an optical phase modulator having a phase modulation amplitude, said modulator coupled to said coil; an oscillator coupled to said modulator said oscillator producing a periodic voltage which controls said phase modulation amplitude; light detection means for detecting and converting said combined light beam into an electrical current, said combined light signal being transmitted through said light source and received by said detecting means coupled to said light source at said back output; an electrical amplifier coupled to said detector for converting said current into an electrical voltage; alternating current voltage amplitude controller means coupled to said amplifier, for controlling said associated light source intensity; and electrical signal processing means coupled to said amplifier for processing said voltage and providing an output signal proportional to the angular rotation rate input of said sensing coil.